This invention relates to a yogurt and to a process for its production and also to a mutant of "Lactobacillus bulgaricus" and to a process for its selection.
Yogurt is obtained by fermentation of milk with a combination of strains of "Streptococcus thermophilus" and Lactobacillus bulgaricus and is present in the form of a gel containing the living strains. Yogurt is a fresh product with a limited shelf life, even under refrigeration. Various processes have been proposed with a view to improving its keeping properties and, in particular, reducing the increase in its acidity, in other words its post-acidification, and the appearance of a bitter taste which adversely affect its organoleptic qualities.
Thus, Japanese Patent Application Kakai No.85256341 describes the production of a mild yogurt using a mixed starter in which the numbers of cells/ml of "L. bulgaricus" and "S. thermophilus" are in a preferred ratio of 1:100. Although the reduction in the relative number of "L. bulgaricus" cells does enable the post-acidification of the yogurt during refrigerated storage to be reduced, it can also produce a reduction in the typically yogurt organoleptic character of the product which is due to the work of the two microorganisms in symbiosis.
U.S. Pat. No. 4,425,366 describes the production of a long-life yogurt using a combination of strains of "L. bulgaricus" and "S. thermophilus" in which the "L. bulgaricus" strain shows low proteolytic activity. However, this document also proposes using a relative number of "L. bulgaricus" cells of the order of 1:100.
U.S. Pat. No. 4,734,361 describes the selection of a strain of "L. bulgaricus" sensitive to low temperatures, i.e. having a low activity at a storage temperature of 10.degree. C. and high activity at a fermentation temperature of 43.degree. C. The strain FERM BP-1041 thus selected was derived from the strain ATCC 11842.
Japanese Patent Application Kokai No. 90053437 describes the production of a yogurt using - in combination with a strain of "S. thermophilus" a strain of "L. bulgaricus" selected for its inability (artificial mutant SBT-0218) or its reduced capacity (natural mutant SBT-0220) to ferment lactose. The starting milk thus has to be supplemented with glucose.
International Patent Application No. WO90/05459 proposes the construction of an "L. bulgaricus" sensitive to low temperatures or to low pH values by selective mutation on "E. coli" of a .beta.-galactosidase gene taken from "L. bulgaricus" and intended for reincorporation in "L. bulgaricus".